curie

A unit that describes the amount of a radioactive substance in terms of the rate at which its atoms are undergoing radioactive decay. Its value was originally set in a fashion that gave ²²⁶radium a specific activity of approximately 1 curie per gram. The unit is usually encountered as the millicurie (a thousandth of a curie) or microcurie (a millionth of a curie). The curie is still in use but is supposed to be replaced by an SI unit, the becquerel; 1 curie = 3.7 × 10¹⁰ becquerel.

The curie was first defined by the International Radium Standards Committee at a meeting of the International Congress on Radiology and Electricity in Brussels in 1910. Among its members the committee included such eminences as Marie Curie, Ernest Rutherford and Frederick Soddy. The primary task of the committee was to define a unit for radioactivity.

The curie was named for the then recently deceased Pierre Curie (1859–1906), French physicist and husband of Marie, and defined as the quantity of “radium emanation” (i.e., ²²²radon, half life 3.8 days) in equilibrium with 1 gram of ²²⁶radium (half life, 1600 years), which requires some explanation. Imagine some ²²⁶radium is put in a sealed bottle. When a radium atom undergoes radioactive decay, an atom of the radioactive gas ²²²radon is produced. Since the gas is also radioactive, eventually the radon atom will decay to produce a third substance. The amount of radon in the bottle will continue to increase as long as the number of radon-destroying disintegrations per second among the radon atoms is less than the number of radon-producing disintegrations among the radium atoms. When enough radon accumulates that those numbers are equal, the radon and radium are said to be in equilibrium, and the amount of radon in the bottle will thereafter be constant.

Then as now the curie is a very large unit in relation to the quantities that workers usually handle. Its size is due to the insistence of Marie Curie at the 1910 meeting that she did not want her husband's name on a unit that represented an infinitesimal quantity.

Marie Curie was asked by the committee to prepare a standard, which she did. The first radium standard, a glass tube containing 21.99 milligrams of anhydrous radium chloride, was left with the BIPM in 1913.

In 1930, the International Radium Standard Commission made the curie the equilibrium quantity of any decay product of radium. In practice, however, workers in the field had begun to use a different definition for the curie, applying it to any radioactive substance. To them, a curie was that quantity of any radioactive substance in which 3.7 × 10¹⁰ disintegrations occur each second (which is the number of disintegrations per second in 1 gram of radium). In 1948, the Committee on Standards and Units of Radioactivity of the National Research Council (United States) recommended that this unofficial definition be made the official definition of the curie. At a meeting in Paris in July 1950, the Joint Commission of the International Council of Scientific Unions on Standards, Units and Constants of Radioactivity recommended the above definition.¹

The curie is typically used in working with radioactive isotopes that are used, for example, as tracers. If a substance has a specific activity of 41 millicuries per gram, in a milligram of it will there will be (3.7 × 10¹⁰ × 0.041, ÷ 1000 =) 1,517,000 nuclear disintegrations per second.

1. National Research Council.
A Glossary of Terms in Nuclear Science and Technology.
New York: American Society of Mechanical Engineers, 1955.

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